US swimmer Gretchen Walsh breaks world record to put Australia on notice following Paris Olympics trials

Australia's swimming squad know they will not have it all their own way at the Paris Olympics, with American Gretchen Walsh breaking the world record for the women's 100 metres butterfly.

A day after the Australian selection trials wrapped up, Walsh kicked off the US equivalent in Indianapolis with a bang, posting a time of 55.18 seconds in her semifinal.

She was more than a half a second under world-record pace at the turn and finished strongly to eclipse the mark of 55.48 set by Sweden's Sarah Sjöström at the 2016 Rio de Janeiro Olympics.

Cheered on by a crowd of 20,689 at the home of the NFL's Indianapolis Colts, Walsh held her hand over her mouth as she looked at the scoreboard in disbelief to see "WR" beside her name.

"I definitely was feeling it," said Walsh, who considers herself a stronger swimmer in the short-course pool.

"I thought I was dying. I didn't know I was going that fast and, apparently, I took it out too fast."

The 21-year-old from Nashville will return for the finals on Monday, Australian time, looking to claim a spot on her first Olympic team.

Five-time Olympic gold medallist Emma McKeon won the women's 100m butterfly at the Australian selection trials in Brisbane with a time of 56.85.

McKeon claimed bronze in the event at the Tokyo Olympics.

As expected, Australia's reigning Olympic champion Ariarne Titmus will face rival Katie Ledecky in the women's 400m freestyle at the Paris Olympics.

Seven-time Olympic gold medallist Ledecky, who was runner-up to Titmus at the Tokyo Games three years ago, won the final at the US trials in a time of 3:58.35.

Titmus clocked 3:55.44 — 0.06 outside her world record — when she won the event at the Australian trials last Monday night.

She later set a world record of 1:52.23 in the 200m freestyle when she edged out training partner Mollie O'Callaghan by 0.25.

Ledecky, who won the 400m freestyle gold at the 2016 Games, will be contesting her fourth Olympics in Paris.

"I'm pretty excited," Ledecky said.

"I never imagined I would go to a fourth Olympic Games."

ABC/wires